This is a topic for cool things that a Bionicle Set can do, but was clearly not designed to do. I’ll start.
Four Shadow Leeches can be crammed into '03 Makuta’s chest despite the lack of Tridax Pod.
Rahkshi can wear Kanohi (01-02 style) on their heads. (Just put the little stud in the hole on top of the head.)
Tarduk can hold onto to almost any surface. (This is due to his Onua claws being held they way they are in the “hand” piece.)
Thornax Launchers can fire Zamor Spheres. (Later exploited in Hero Factory.)
Vakamas disk launcher can be used as a jetpack.
Disk launchers placed at the back with trans disks create a halo effect on the figure.
Friction adders can be used to extend t and create a neck on sets that have heads very close to their torsos.
Bohrok shields are great as torso armor for metru builds.
Knights Kingdoms limb armor is great as armor for figures.
Round studs are great for filling up unused technic holes.
Lewa Mata already has the strongest attack function of any Toa, but you can make it even stronger by attaching his left hand to the pin hole at the base of his axe. Possibly intentional, though it does tend to knock his mask off.
Pohatu Mata can hold his Koli ball by plugging the studs into the holes of his arms.
I don’t know if this is intentional or not but Gresh can use his shield as blades and the Piraka can turn their weapons around, also if you wanted you could rebuild Pohatu Nuva to make him look like the other Toa Nuva and move his arms instead of his legs.